First, this is not a "Performance Tuning" related question, rather general repair should be put in the appropriate subforum for your generation of vehicle.
This also helps because you didn't mention your model year or which engine. Sometimes it doesn't matter but other times, you have to know the details to know it doesn't matter.
Trace all vac lines on the intake, that they're plugged in there and the other end of each line.
I suppose it's possible an IAC's internal airflow could whistle, but calling it a leak, well sort of as it could need cleaned out. Pull it off and generously spray carb or intake cleaner on it, let it sit, spray some more and use a cotton swab or similar to clean off the plunger hat inside where it seals against the cavity on the IAC body. It's spring loaded so you may need a pick or needle nose pliers to hold the plunger down to get access to it.
If your IAC also has a plastic cap on the side, gently pry it off and rinse out the two felt filter pads in it with solvent, and shoot some solvent down in the hole that those cover, and again shake out excess solvent.
After cleaning and shaking out excess cleaning solvent, I like to put a drop of motor oil on the shaft where it meets the bushing, and prop up the whole IAC so that gravity causes the oil to seep into the bushing, leaving it like that for a few hours before reinstalling on the vehicle.
While the IAC is off, note the state of the bottom. On some if not all engines there is a silicone rubber gasket. It and the mating surface on the IAC body should be clean. Some might have a paper gasket too which isn't really needed if the silicone gasket is intact, but if it's there it is another source of a leak and probably shouldn't be reused.
About the misfire. Pull the OBDII codes, look them up and post them here. Is it only one cylinder? If so you might pull the spark plug and take a look, just to verify it's in good shape and what it looks like, and you could confirm that you're getting spark on it while engine is running. You might have a waste spark system on the (whichever?) engine such that if one plug isn't making spark, the opposing bank plug on the same ignition coil won't make spark either.
The OBDII codes could be important to note other things like running too lean. Perhaps you have an intake manifold leak. Running without the (electric) cooling fan plugged in, or belt off a mechanically driven fan, (for only a few minutes at most so it doesn't overheat), you can use a cigar to smoke test the intake for leaks, or spray starter fluid or carb cleaner, etc to see if it seems like a particular area of the intake is leaking.
Try not to drive it much until fixed, excess unburnt fuel resulting from misfire can ruin a catalytic converter if exposed to it for too long. So far I've only mentioned leaks since you mentioned the whistle sound. Once a vehicle gets older there could be more than one thing wrong or a sound is just a coincidence.
For example you could have a clogged fuel injector causing misfire and just observed the whistle because you started looking at the engine closer, or even a warped intake, warped head, cracked head or cylinder wall. These are less likely unless there was an extreme overheating situation but might be detected with a cylinder compression test or leak down test.
Sometimes its easiest to try the simple quick things first. For example if it seems like IAC is leaking, see if the bolts for it have worked loose, but otherwise I'd start with pulling the OBDII codes.